I'm not certain it's been 20 + years since I had classes on it. But water on a magnesium fire is a big mistake it will explode and intensify greatly in strenght. It will burn underwater as well. It reaches temps into the 5000F range. One way to fight it is to fog it but I wouldn't want to try it. You can do the same with a deep fryer but you have to know exactally what you are doing. On ships protocol in a magnesium fire Or Delta fire is jetison over the side as it will burn to the bottom if you don't. As one fireman said in here the only thing workable usually is sand.
As I said, magnesium will decompose water. Unless you're trying to burn it underwater, though, that's different from "producing its own oxygen". BTW, does jettisoning a burning aircraft pose any risk of a hydrogen explosion above the surface, or is the hydrogen too diffuse at that point to pose a problem?